BOSS DD3 STAYS ON AND WILL NOT TURN OFF

Use 1/4" plug cables or XLR-1/4" cables with the fx loop, depending on the type of connector of your mixer. Don't use batteries, they don't last. Use a type 2 (for noise reduction) 9V wall outlet adapter.

DId you remove the plug from the input jack to turn the pedal off? If so then you may have a bad input jack.

Make sure you are NOT using a TRS (stereo or three conductor cable) type cable. PLugging into the input should turn the pedal ON, not off. The switch on the jack may be broken or soldering to the circuit board may be broken OR worst case, there is a crack in the circuit board.

Plug your guitar into the input and a cord from one or both outputs to the inputs of whatever you are using. Meant to be run in stereo for best sound. Two guitar amps?

I hope something here might help:http://www.guitarristas.info/foro/archive/f-51.html I regret I only speak English.

The schematic is here:http://www.vintagesynth.com/roland/xp60.php Take note though that the design was changed for the newer DD3. Here is a link to a forum with links to later DD3 service notes:http://music-electronics-forum.com/t15879/. Start checking transistors at the input. Check voltage readings on pins on the I.C.s to see if they match the supply voltage at IC10 . If you get voltage readings on pins that aren't supposed to have readings there is a good chance that the I.C. you are testing is shorted. Also check the diodes as you proceed.

You are probably overdriving the pedal. Reduce the amplitude INTO the pedal and AMPLIFY it more AFTER the pedal.

When digital conversion is done from the input and the input exceeds the input voltage capabilty of the A-D convertor, it effectively "flattops" just like your overdrive on a guitar amp.

Your guitar may be very "hot" having high output. One needs to keep the signal levels in-bounds of what the hardware can handle at EVERY point.

Have you tried a new battery?

If you have experience with electronics then open it up and start testing all the components with a digital multimeter to find the failed component(s).

Unfortunately the newer DD3's are harder to fix then they once were. There is no place to point you other than just to start testing everything. Hard to say what has failed. The signal into the input of the circuit may simply be shorting to the output. Maybe the switch or switching circuit has a fault.

Here is the schematic as a reference: http://www.8bitsindgenug.net/boss_dd2.png

With everything switched off. I will surmise that you are using the onboard battery with the foot pedal and not a power supply.
1. Plug lead from guitar/instrument into INPUT side of pedal
2. Plug lead from OUTPUT side of pedal into amplifier INPUT.
3. Turn amplifier volume to about 10 o'clock - to stop blowing your ears out.
4. Turn volume up on guitar to acceptable level.
5. press foot switch on pedal and adjust pedal controls to suit your type of distortion.
6. Play like Hendrix.
Good Luck
ps Just as an after thought rechargeable 9volt batteries are the way to go. I've been using them for years as I always get caught up in the power cable.
If you're going to add another pedal like a Phaser, Compressor, Flanger or what just use small connect leads between each pedal but always, always put the compressor pedal last in the line.

If you have experience with electronics you could take it apart and use a multimeter. My guess would be one of the delay chips but its something hard to diagnose without taking it apart. An audio tech could help you out. Chances are it would be a pretty cheap fix. Unfortunately Boss pedals are almost entirely surface mount so replacing a component or two takes more skill than the average pedal.